Final answer:
The force of sliding friction on the box is 85 Newtons, as this opposes the applied force, allowing the box to slide at constant speed according to Newton's first law of motion.
Step-by-step explanation:
When a box slides along the floor with a constant speed and is being pushed with a force, the force of sliding friction on the box is equal to the force being applied in the opposite direction. This is because, according to Newton's first law of motion, an object in motion will remain in motion with a constant velocity if the net external force acting on it is zero. Therefore, if a box is pushed with a force of 85 Newtons and it moves at a constant speed, the force of sliding friction must be 85 Newtons to oppose the push and result in no acceleration, maintaining the current state of motion.